Someone leaked a prototype of the Japanese version on the Internet a long time ago (CRC32: 3CA1EC46). I'll admit I had to use an infinite lives cheat for this one (7E1FB203), as its "second level of Contra"-style gameplay is a bit out of my league.

The gameplay for the prototype is fairly close to the final version. I think the prototype is somewhat harder than the Japanese commercial release. The prototype doesn't have all of the powerups of the final version (such as the "powerup" that gives you 50 shots of a non-damaging pellet gun), and it doesn't include point bonuses for killing several enemies in a row. The level names in the prototype are not the same as the final version. Once you beat two of the selectable stages (any two), the game over screen pops up.

Prototype - title screen. Note the large "SAMPLE"

Final - title screen

Prototype - intro to the first level. The stage is called "Dangerous Town"

Final - The stage is called "Carson City"

Prototype - first selectable state, called "Devils Mountain"

Final - The stage is called "Desolation Canyon"

Prototype - second selectable state, called "Ordnance Factory"

Final - The stage is called "Ammunition Depot"

After beating two of the selectable stages, the Game Over screen greets you.

I found a second ROM image that was labelled as "BETA" in my set (CRC32: CEAAC33D). I tried it out, but couldn't find anything that distinguished it from the final Japanese version. There wasn't much in the binary to distinguish it either, except that some of the areas that were blank in the final version had some non-blank information. Whether this is a hack, a beta or a bad dump, I could not tell you.

Though the exact origin of these ROM images is not known, they were probably leaked onto a BBS originally. The oldest version of GOODSNES that I have (0.998, released in May 2000) has both of the above ROM images. Looking through my BBS archive, I found a trainer for the sample prototype, released by the group "Cyberforce", dating to June 1994. This suggests the prototype has been distributed since at least that time. My archive doesn't have any ROM images, though.